1. Field of the Invention
The present invention relates to a position control system for electro-hydraulically operated turbine inlet valves.
2. Description of the Prior Art
The inlet valves, which control the admission of fluid under pressure to operate the turbine of an electric power plant, typically, are positioned by an hydraulic valve actuator. The hydraulic valve actuator is controlled by a servo valve that admits hydraulic fluid under pressure to the actuator in accordance with the value of an electrical signal generated by a turbine control system. Mechanically coupled to the actuator is a linear variable differential transformer (LVDT) that generates through a demodulator an electrical feedback signal coresponding to the actual position of the inlet valve. This feedback signal is summed or compared with the valve control electrical signal to insure that the inlet valve or valves are operated to the exact position required by the turbine control system.
The signal from the turbine control system is typically an analog DC signal that varies from 0 to +10 volts, for example, with the minimum voltage requiring a fully closed valve position; and the maximum voltage requiring a fully open valve position. The feedback signal at the output of the demodulator is also an analog DC signal generated by the LVDT that varies from 0 to +10 volts, for example, with the minimum voltage representing an actual closed position of the valve, and the maximum value representing an actual fully open position of the valve. The system includes a porportional plus integral controller that responds to an error signal that is caused by a change either in the control signal or the feedback signal to change its output signal for moving the valve until the demodulated LVDT signal when summed with the control signal results in an effective no error signal to the input of the controller.
Prior to the present invention, such valve positioning control systems, included ground connections, which could at times cause a difference in ground potential that would result in undesirable valve movement, particularly in an enrivonment where there was a substantial prevalence of electrical noise. Also, such systems included a proportional plus integral controller that was so constituted that an adjustment of the system resulted in the adjustment of both the proportional and the integral or reset parameters. Thus, it was difficult to adjust the system such that a relatively large error signal did not cause overshooting of valve position without an accompanying delay in the valve moving to the desired position. Further, in operating the valves from a fully open to a fully closed position, a delayed response due to the saturation of the system servo amplifier could occur.
Inlet valves, such as steam inlet valves for turbine power plants, have non-linear position versus flow characteristics, (i.e., for example, a 20 to 30% valve position may provide an 80% steam flow); and this nonlinearity may vary in accordance with the type of valve and with the upstream and downstream pressures at which the system operates. It is desirable in turbine control systems to operate the valves in accordance with the desired steam flow through the valves instead of a desired valve position. Depending on the type of control system, this is accomplished by either characterizing the input or control signal, or by characterizing the position feedback signal from the LVDT in accordance with the predetermined curve of steam flow versus valve lift position. For those systems that characterize the input or control signal, the feedback signal is linear; (that is, a 60% input or control signal requires a 60% valve position, for example). However, in those systems that characterize the feedback position signal, the input signal is not characterized; (i.e., for example, an 80% flow or input signal may move the valve to only a 30% open position). This characterization of feedback position is accomplished, typically, by a function generator or position characterizer that generates a predetermined output signal in response to a particular input signal from the LVDT. The characterization is affected by one or more line segments, i.e., an output signal is a certain linear function of the input signal over one range of input values, and then at a break point value, the output signal becomes another linear function of the input signal. Thus, the curve of flow versus position can be approximated by the line segments. It is evident, therefore, tht the more line segments involved in the characterization, the greater the accuracy or approximation of the curve.
However, because of the inherent drift characteristics of the electronic components of the system, it was desirable to minimize the number of line segments utilized for curve approximation. Each one of the line segments would be dependent on the adjacent one, i.e., if one line segment should drift, the break point of the next line segment would be at a different point and such error would be multiplied. With a number of line segments, this drift could cause the resulting curve to be quite dissimilar to the desired flow versus position characterization. Also, such drift could cause the valve position to be satisfied by more than one point on adjacent line segments. Also, in such systems it is difficult to calibrate the valve control system to provide for the desired curve relationship.
In view of the above, it is desirable to provide an improved valve position servo system, the operation of which does not cause undesirable valve movement. One way of accomplishing this result is by signal conditioning the input and feedback signals with differential amplifiers to provide a high common mode noise rejection and common mode voltage range. Also, to provide for more accurate and versatile valve control, it is desirable that such a system provide for independent adjustment of the following and converging errors of the proportional plus integral controller. Further, such a system should insure fast valve response at all times, even from a fully open to a fully closed position. This may be accomplished by preventing the systems servo amplifier from saturating while in a fully open position.
It is further desirable, that such a servo control system may be used with turbine control systems that require both linear and position characterization feedback; and for control systems requiring position characterization feedback, such a system should be capable of utilizing a number of line segments for more accurate representation of the valve position versus flow curve without the multiplying effects of drift. Also, such a system should be versatile and useful for various types of valves requiring different characterization curves, and which are readily adjustable for particular applications.